SIL-233 3(2-3)

POLUSI TANAH DAN

KONTAMINASI AIRTANAH

2. Karakteristik fisik, kimia dan biologi tanah

Pengajar:

Prof.Dr. Budi I. Setiawan

Dr. Satyanto K. Saptomo

LUAS PERMUKAAN SPESIFIK

• Luas Permukaan Spesifik (Specific Surface Area/SSA) merupakan sifik fisik fundamental dan intrisik tanah yang berkaitan dengan fenomena penting seperti pertukaran kation, retensi dan pelepasan bahan-bahan kimia (termasuk nutrisi tanaman, polutan potensial), retensi air, sifat mekanika sperti plastisitas, kohesi dan kekuatan.

• Berikut 3 ekspesi matematika untuk menghitung LPS: – am= As/Ms;

– av= As/Vs;

– ab= As/Vt;

KA

RA

KT

ER

IST

IK T

AN

AH

LUAS PERMUKAAN SPESIFIK

KA

RA

KT

ER

IST

IK T

AN

AH

SSA, KUBUS BERBAGAI UKURAN

KA

RA

KT

ER

IST

IK T

AN

AH

Sisi Luas 1 Muka Luas Permukaan Volume Area per Volume

1 m 1 m2 6 m2 1 m3 6.0 m−1

2 m 4 m2 24 m2 8 m3 3.0 m−1

4 m 16 m2 96 m2 64 m3 1.5 m−1

6 m 36 m2 216 m2 216 m3 1.0 m−1

8 m 64 m2 384 m2 512 m3 0.75 m−1

12 m 144 m2 864 m2 1728 m3 0.5 m−1

20 m 400 m2 2400 m2 8000 m3 0.3 m−1

•Semakin kecil ukuran kubus, semakin besar luas permukaan spesifiknya.

•Hal ini berlaku juga untuk semua bentuk padatan.

SSA PARTIKEL TANAH• Apabila suatu kubus dengan sisi-sisi l diiris dengan

sebanyan (n-1) kali dengan ketebalan =l/n untuk mendapatkan n buah lempeng, maka bila a dan b adalah panjang dan lebar lempeng tersebut luas permukaan spesifiknya adalah:

babaab

baabs

sss

11121

22

KA

RA

KT

ER

IST

IK T

AN

AH

• Apabila <<a dan <<b

• Apabila <<a dan =b

s

s

s

s

4

2

SSA PARTIKEL TANAH

KA

RA

KT

ER

IST

IK T

AN

AH

SSA PARTIKEL TANAH

• Kubus dan bola memiliki SSA

yang sama bila diameter dan

panjang sisinya sama.

• Semakin tipis partikel,

semakin besar SSA

• Semakin besar SSA semakin

besar terjadi aktivitas

permukaan dan aktivitas

koloidal.

• Total SSA adalah jumlah dari

fraksi (f) dikalikan SSA dari

masing-masing partikel

KA

RA

KT

ER

IST

IK T

AN

AH

ii isfs

• Ukuran partikel tanah beragam dari sedikit lebih besar dari ukuran molekuler sampai batu, dengan partikel besar membentuk kerangka dan molekul kecil mengisi ruang kosong disekeliling titik kontak dan menutupi permukaannya.

Interaksi Padatan dan Cairan

Terjadi kontak antara fasa cair dan fasa padat dalam pori tanah. Pada bidang kontak ini gaya listrik alami menyebabkan fenomena pengembangan, pengkerutan, agregasi, flocculation dan dispersi yang akhirnya akan mempengaruhi transport udara dan larutan dalam tanah.

Mineral Liat (Clay)

• Liat adalah fase padatan dalam tanah dengan

diameter kurang dari 2 µm. Didalamnya

biasanya didapati colloidal clay yaitu sub-fraksi

yang lebih halus dari 0. 2 µm.

Clay

• Terdiri dari silika dan

alumina

• Terbentuk dari 2 lembaran

(sheet) silika dan satu

lembaran alumina (2:1)

atau satu lembaran silika

dan alumina (1:1)

• Contoh : Kaolinite,

bermacam-macam mika,

illite, chlorite, vermichullite,

montmorilonite, allophanes.

Muatan Listrik Permukaan

Mineral Liat

• Dalam pembentukan mineral liat,

lembaran-lembaran tetrahedral dan

octahedral berkembang unit per unit.

• Si, Al dan Mg jarang terdapat dalam

perbandingan yang ideal. Ion-ion A13+

yang berlebih dapat menngisi posisi

Si4+, demikian juga Mg2+ dapat

mengisi bila terjadi kekurangan A13+ .

• Proses ini menyebabkan terjadinya

kelebihan muatan negatif yang

bermanifestasi sebagai densitas

muatan permukaan spesifik.

Muatan Listrik Permukaan

Mineral Liat• Subtitusi isomorphous : Penggantian satu kation dengan

kation lain yang berukuran sama (Si4+ oleh Al3+ ; A13+

oleh Mg2+); tidak terlalu mempengaruhi bentuk kisi-kisi

kristal (crystal lattice). Setelah mineral liat terbentuk,

sangat sulit untuk mengeluarkan kation subtitusi tanpa

merusak lattice.

• Pada beberapa mineral liat tidak terjadi subtitusi

isomorphous sehingga secara elektrik tetap netral

kecuali ada sumber muatan listrik lain.

• Dibawah pH = 7, tepian partikel liat memiliki muatan

positif.

Adsorption* and dissociation **

of counterions

• Muatan negatif liat akan terkompensasi secara natural

oleh muatan positif yang ekivalen dari kation-kation:

seperti Na +, K+ , H + , Mg 2+ and Ca 2+, yang akan

diserap di permukaan lempeng atau tersebar di larutan

yang mengelilinginya.

• Kation-kation melawan muatan listrik negatif pada plat

mineral liat dan menghadap sekelilingnya dan disebut

juga sebagai counterions.

*Adsorption is the adhesion of molecules of gas, liquid, or dissolved solids to a surface** Dissociation in chemistry and biochemistry is a general process in which ionic compounds (complexes, or salts) separate or split into smaller particles, ions, or radicals, usually in a reversible manner

Adsorption and dissociation of

counterions• Apabila lempeng liat kering dibasahi dengan air kation yang ter-

adsorp dengan lemah akan mengalami dissosiasi (dissociation) dari permukaan lempeng dan yang kuat ter-adsorp akan membentuk lapisan tipis di permukaan lempeng.

Adsorption and dissociation of

counterions

Permukaan lempeng liat yang

bermuatan dan counterion yang

mengelilinginya disebut diffuse

electrical double layer. Merupakan

terminologi yang menunjukan

pemisahan lempeng bermuatan

negatif dan counterion positif. Counter

ion dibedakan lagi menjadi fixed

layer/Stern layer dan diffuse layer.

Distribusi ion dekat permukaan

bermuatan negatif

Swelling and Shrinkage• Pada kadar air rendah larutan

tanah disekitar partikel liat akan berupa lapisan film tipis. Double layer akan mencegah lapisan ini untuk mengembang. Dalam kondisi ini konsentrasi kation menjadi lebih tinggi dan memiliki kapasitas penyerapan air secara osmosis.

• Apabila air terserap, partikel-partikel akan bergerak mendorong satu sama lain sehingga terjadi gaya pengembangan /swelling. Besar pengembangan ini akan tergantung dari double layer i.e. tergantung pada jenis liat nya.

Interaksi antara padatan

Kondensasi lempeng

• Bila jarak antar lempeng-lempeng liat sangat berkurang akibat penghilangan air counterion dalam double layer akan dipaksa masuk ke tengah-tengahnya. Muatan positif dan negatif akan saling mendekat dan tarik menarik.

• Pada liat dengan konsentrasi Ca2 +

(or other polyvalent ions) yang tinggi, tarik-menarik ini akan cukup kuat sehingga menjadi kondisi yang stabil yang disebut kondensasi plat yang tidak memungkinkan lagi air dapat terserap

Interaction between soild phase• Flocculation

• The edges of clay platelets have a positive electrical charge at pH below 7. If the extent of the double layer is small, the positively charged edges may approach the negatively charged surfaces of the platelets sufficiently to form weakly bonded floccules. This process is called flocculation. Floccules are loose combinations of clay particles similar to *card-house*arrangements (Figure 1.8). Since the stability of these arrangements decreases with increasing extent of the double layer, a Ca-clay will flocculate at a lower salt concentration of the soil solution than a Na-clay.

• Clays suspensions probably initially flocculate in card-house arrangements. With slow drying, the floccules tend to orient and stick together in the same manner as with plate-condensation.

Interaction between soild phase

• Peptisation, dispersion or deflocculaltion is the reverse process of flocculation, namely the separation into primary particles. This can be achieved either chemically or mechanically.

• When a flocculated clay is dried, it forms a sediment with extensive cracks. It also tends to be crumbly. When this sediment is rewetted, the crumbs as well as the sediment as a whole are stable. When a dispersed clay is dried, it forms a hard crust. If the wetting and drying cycle is repeated, hard, large clods are formed. When the sediment is rewetted, it forms a sticky mud without structure. This is typical of Na-clays, but not of Ca-clays.

Interaction between soild phase

Cementing agents

• The primary soil particles within an aggregate may be bound together by certain substances called cementing agents. The main cementing agents are organic matter, silicate clays, lime and sesquioxides.

• Humus, which is colloidal organic matter like clay, adsorbs cations. If humus contains a high proportion of Ca2+- and other divalent cations, its long polymer chains can form bonds with each other and with the mineral components of the solid phase. It also binds clay domains to quartz, which is the primary mineral component of silt and sand. In this way, a stable 'clay-humus complex' is formed, resulting in aggregates. In soils rich in Na+- (alkali soils) or H+-ions (acid soils), the bonds are unstable and the humus dissolves.

Interaction between soild phaseCementing agents

• Silicate clays, too, may cement particles together, but their binding effect is much smaller than that of humus. The degree of cementing and the kind of aggregates formed, vary with the electrical charge, its distribution on the clay particles and the type of clay involved.

• Other cementing agents for sand and silt are lime (mainly CaCO3) and sesquioxides (Al- and Fe-oxides). Lime, when it precipitates around contact points between soil particles, acts as a cement. The binding effect of iron oxide is doubtful, but aluminum oxide is probably effective. Finally, soil organisms also may keep soil particles together by their activity and sometimes by their byproducts.

Interaction between soild phase

Soil structure and structural stability

• The spatial arrangement or clustering of primary soil particles into secondary units, called aggregates or peds, is known as soil structure.Soil structure influences water transport, soil temperature, air transport and mechanical impedance of soil to seedling emergence and root penetration. It is nearly impossible to measure soil structure directly. The spatial arrangement and orientation of soil particles and soil pores as seen in thin sections, probably represents soil structure the closest. Soil structure is often, somewhat arbitrarily, described by the sizes and shapes of aggregates. It is also characterized by porosity and pore-size distribution These are an indication of the soil ability to retain water, to allow water to infiltrate and to make water available to crops. One can also measure parameters such as soil air permeability, infiltration rate, bulk density, aeration porosity and penetration resistance. Though indirectly, these measurements all render information about soil structure.

• Processes such as plate condensation and flocculation of clays and humus enhance structure formation, but are by themselves not sufficient. Structure and its stability depend upon the processes of formation, as well as the presence of adequate binding agents. If sufficient amounts of cementing agents are present, even sandy soils can form stable fabrics with many large and small pores produced by soil fauna, roots or frost.

Interaction between soild phase

Soil structure and structural stability .

• Structural stability of aggregates is as relevant as structure itself. It is the resistance of soil structure to mechanical and physico-chemical destructive forces. This resistance is determined by the attractive forces between the soil constituents under various conditions. For instance, lack of stability of soil structure may seal the soil surface and reduce water infiltration, hamper aeration and impede emergence of seedlings. The process by which dry soil aggregates disintegrate upon wetting is called slaking. This may be aggravated by air locked inside aggregates, while a dry soil is being flooded

• or infiltrated. The pressure of occluded air may then build up such that the aggregates explode. When the disintegrated aggregates dry, a soil crust may be formed.

• Other physico-chemical forces include swelling, shrinkage, flocculation and dispersion. Frost action may also improve soil structure. Growing ice lenses displace soil particles and form large pores. The water being accumulated in the ice lenses causes decreasing water contents elsewhere, often leading to plate condensation, flocculation, etc.

Quest

1. Hitung luas area spesifik untuk partikel liat yang berbentuk lempeng dengan ketebalan = 1nm, panjang dan lebar = 20 nm dan masajenis 2750 kg/m3

2. Terjmahkan slide-slide terakhir yang berbahasa Inggris ke dalam bahasa Indonesia (Optional)

• Kirim jawaban anda via email ke saptomo@ ipb. ac. Id sebelum kuliah Selasa 2 Maret 2010.